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On Wednesday, Khalid Masood drove a sport utility vehicle on a sidewalk on Westminster Bridge, killing four people and injuring dozens more. A police officer fatally shot Masood after he left the vehicle.

Rowley said 16 searches at various locations have finished but that five others were ongoing. Officers have seized 2,700 items, including "massive amounts" of computer data, and have had contact with 3,500 witnesses to the attack, many of them of different nationalities, he added.

Authorities are investigating how the perpetrator, who used multiple aliases, became radicalized. He was born Adrian Russell Ajao but also used the name Adrian Elms, police said Friday.

Rowley said: "Clearly that's the main line of our investigation — is what led him to be radicalized. Was it through influences in a community, influences from overseas or through online propaganda?"

He appealed for anyone who knew Masood well or was aware of his recent movements to get in touch with authorities.

Born in Kent, Masood had previous convictions, including some for violent offenses, but none for terrorism, police said. His most recent conviction was in 2003 for possession of a knife.

The Royal Embassy of Saudi Arabia issued a statement Saturday saying Masood visited Saudi Arabia from November 2005 to November 2006 and from April 2008 to April 2009, when he worked as an English teacher. He also obtained a visa and visited Saudi Arabia from March 3 to 8, 2015, the statement said.